1. Field of the Invention
The invention relates to a process for producing metal foams of controlled structure and to the metal bodies in foam form which are obtained in this way.
2. Description of the Related Art
The prior art for the production of metal foams substantially comprises five basic procedures:    1. the compacting of metal powders with suitable blowing agents and heating of the preforms obtained in this way to temperatures which are higher than the liquidus temperature of the metal matrix and higher than the decomposition temperature of the blowing agent used;    2. dissolving or blowing of blowing gases into metal melts;    3. stirring of blowing agents into metal melts;    4. sintering of metallic hollow spheres;    5. infiltration of metal melts into filler bodies, which are removed after the melt has solidified.
Regarding the first procedure, DE-A-197 44 300 deals with the production and use of porous light metal parts or light-metal alloy parts, the bodies which have been compressed from a powder mixture (light-metal or Al alloy and blowing agent) being heated, in a heatable, closed vessel with inlet and outlet openings, to temperatures which are higher than the decomposition temperature of the blowing agent and/or melting temperature of the metal or of the alloy.
With respect to the second procedure, JP 03017236 A describes a process for producing metallic articles with cavities by dissolving gases in a metal melt and then initiating the foaming operation by suddenly reducing the pressure. Cooling of the melt stabilizes the foam obtained in this way.
WO 92/21457 teaches the production of Al foam or Al alloy foam by blowing in gas beneath the surface of a molten metal, abrasives, such as for example SiC, ZrO2 etc., being used as stabilizers.
Concerning the third procedure, according to the teaching given in JP 09241780 A, metallic foams are obtained with the controlled release of blowing gases as a result of the metals initially being melted at temperatures which lie below the decomposition temperature of the blowing agent used. Subsequent dispersion of the blowing agent in the molten metal and heating of the matrix to above the temperature which is then required to release blowing gases leads to a metal foam being formed.
Regarding procedure 4, the production of ultralight Ti—6Al—4V hollow sphere foams is based on the sintering, which takes place at temperatures of ≧1000° C., of hydrated Ti—6Al—4V hollow spheres at 600° C. (Synth./Process. Lightweight Met. Mater. II, Proc. Symp. 2nd (1997), 289-300).
With respect to procedure 5, foamed aluminum is obtained by, after infiltration of molten aluminum into a porous filler, by removal of the filler from the solidified metal (Zhuzao Bianjibu (1997) (2) 1-4; ZHUZET, ISSN: 1001-4977).
Furthermore, components with a hollow profiled section are of particular interest for reducing weight and increasing rigidity. DE-A-195 01 508 deals with a component for the chassis of a motor vehicle which comprises die-cast aluminum and has a hollow profiled section, in the interior of which there is a core of aluminum foam. The integrated aluminum foam core is produced in advance by powder metallurgy and is then fixed to the inner wall of a casting die and surrounded with metal by die-casting.
When assessing the prior art, it can be observed that the processes, which provide for preliminary compacting of preforms and which contain blowing agent, are complex and expensive and are unsuitable for mass production. Moreover, a common feature of these processes is that the desired temperature difference between the melting point of the metal, which is to be foamed, and the decomposition temperature of the blowing agent used should be as low as possible, since otherwise disruptive decomposition of blowing agent takes place even during compacting or later in the melting phase. This observation applies in a similar way to the introduction of blowing agents into metal melts.
The sintering of preformed hollow spheres to form a metallic foam is at best of academic interest, since even the production of the hollow spheres requires a complex procedure.
The infiltration technique has to be considered in a similar way, since the porous filler has to be removed from the foam matrix, which is a difficult operation. The dissolving or blowing of blowing gases into metal melts is not suitable for the production of near net shape components, since a system comprising the melt with occluded gas bubbles is not stable for a sufficient time for it to be processed in shaping dies. The mechanical properties of metal foams are substantially—in addition to the selection of the metal or alloy used—determined by their structure.
However, the linked procedures which take place during the production of porous metal bodies often—in particular in the case of the method which is based on the use of chemical blowing agents—do not provide the desired result of a uniform metal foam which has globular cells of similar dimensions. Associated with this is, for example, a lack of isotropy of the bulk density, which could be desirable for the subsequent function of the metal foam in numerous structural components. Instead, there are irregularities, in the form of thickened zones in the metal body (for example a pronounced foot and/or edge zone formation and/or associated cavities which result from individual gas bubbles combining with one another as a result of the cell membranes being destroyed). At the same time, the occurrence of irregularities of this nature may indicate a relatively inefficient utilization of blowing agent.